In particular, the emulated imaging system may be a photolithographic scanner used to produce microelectronic circuits. For this purpose, photolithographic masks are used whose structures are imaged onto wafers coated with photoresist, such that the resist is exposed and the structure on the wafer is modified at the exposed locations.
Emulation imaging systems are used to emulate such a photolithographic scanner. However, whereas the scanner images a reduced-size image of the mask's structure onto the carrier to be exposed, an emulation imaging system is used in mask inspection systems, in which case an enlarged image of the structure is imaged onto a detector. While both systems have the same numerical aperture on the mask side, they differ on the image side. The image-side numerical aperture of a mask inspection system, on the one hand, is approximately 0. On the other hand, increasingly smaller object structure sizes require increasingly greater image-side numerical apertures of 0.8 and more in the photolithographic scanner. This leads to deviations in the images of the mask inspection system and of the photolithographic scanner, which are no longer negligible, particularly in cases where the ratio of the numerical aperture of the scanner to the refractive index in the photoresist is greater than 0.8/1.7. These deviations, or defects appearing in the scanner system, respectively, also include the so-called apodization. The transmission of light in the photoresist as a function of the angle of incidence is not constant, but increases towards great angles of incidence.
In the prior art, the image-side differences between the photolithographic scanner and the emulation imaging system for mask inspection only play a secondary role. However, in the semiconductor industry, the use of immersion systems for manufacturing wafer structures of less than 65 nm is favored for the future. By applying an immersion liquid onto the wafer, numerical apertures NA>1 are achieved on the image-side, allowing smaller structures to be generated at the same wavelength. Thus, for example, using water as the immersion liquid and illumination with light at a wavelength of λ=193 nm, a maximum numerical aperture of 1.4 can be achieved. Even greater numerical apertures can be achieved using other immersion liquids. A reduction factor of 1:4 requires mask structures of 260 nm or 180 nm, respectively, for wafer structures of 65 nm or 45 nm, respectively. Thus, the mask structures are within the range of the imaging wavelength of currently 193 nm. In the case of smaller structures, the analysis of defects in the masks to be used is becoming increasingly important. One example of a mask inspection system suitable for analysis is the AIMS™ (Areal Imaging Measurement System) of Carl Zeiss SMS GmbH. A small area of the mask (site of the defect) is illuminated and imaged under the same conditions of illumination and imaging (wavelength, numerical aperture) as in the photolithographic scanner. In contrast to the photolithographic scanner, however, the image generated by the mask is enlarged and imaged onto a CCD camera. The camera sees the same image as the photoresist on the wafer. Thus, the aerial image can be analyzed without complicated test prints. In this system—just like in the other systems known in the prior art—the deviation in apodization with respect to the photolithographic scanner is not taken into consideration, because the numerical apertures are still so small that the deviations have not played a role so far. However, since the use of larger apertures on the mask side in the photolithographic scanner may be expected in the future, these deviations are becoming more important.